As is well known, there have been proposed various kinds of magnetic detection apparatuses (e.g., refer to Patent Document 1) that each have a bridge circuit for converting a change in the resistance value of a magnetoelectric conversion element, caused by a change in a magnetic field, i.e., a magnetic flux density, into a voltage and then outputting the voltage and that each detect the traveling direction and the like of a magnetic moving body, which magnetically provides an effect to the magnetoelectric conversion element, based on the output voltage of the bridge circuit. Such a magnetic detection apparatus is utilized, for example, as a vehicle rotation sensor that detects the rotation direction and the rotation speed of a vehicle engine.
A conventional magnetic detection apparatus disclosed in Patent Document 1 includes
a first bridge circuit in which respective electrodes are formed at both ends of magnetoelectric conversion elements arranged in such a way as to face a magnetic moving body that travels,
a second bridge circuit in which respective electrodes are formed at both ends of other magnetoelectric conversion elements that are arranged at positions different from those of the magnetoelectric conversion elements in the first bridge circuit and face the magnetic moving body,
a first differential amplification circuit that amplifies a sinusoidal output voltage of the first bridge circuit,
a second differential amplification circuit that amplifies a sinusoidal output voltage of the second bridge circuit,
a first comparison circuit that compares the output signal of the first differential amplification circuit with a predetermined threshold value and then generates a rectangular-wave output signal,
a second comparison circuit that compares the output signal of the second differential amplification circuit with a predetermined threshold value and then generates a rectangular-wave output signal,
a first output circuit that drives a first switching device, based on the output signal of the first comparison circuit, and
a second output circuit that drives a second switching device, based on the output signal of the second comparison circuit; the conventional magnetic detection apparatus generates a first output signal having a rectangular-wave shape, based on the operation of the first switching device, and generates a second output signal having a rectangular-wave shape, based on the operation of the second switching device.
There exists a phase difference of approximately 90° between the respective output voltages of the first and second differential amplification circuits; therefore, at a rising time of the output signal of the first comparison circuit, the output voltage of the second differential amplification circuit is at the bottom position or the peak position thereof, and at a falling time of the output signal of the second comparison circuit, the output voltage of the second differential amplification circuit is at the peak position or the bottom position thereof; thus, the traveling direction of the magnetic moving body can be determined based on the mutual relationship between the first and second output signals.
However, when for example, due to a productional or thermal variation in the characteristics of a magnetoelectric conversion element, there occurs an offset between the output voltage of the first differential amplification circuit and the output voltage of the second differential amplification circuit when the temperature or the like of the environment where the magnetoelectric conversion element is disposed changes; then, eventually, there occurs a variation in the respective falling times or the respective rising times of the first output signal and the second output signal. As a result, the detecting accuracy may be deteriorated.
Accordingly, the foregoing conventional magnetic detection apparatus is configured in such a way that at the falling and rising times of the output signal of the first comparison circuit, the value of the output voltage of the second differential amplification circuit is converted into a digital value, that at the falling and rising times of the output signal of the second comparison circuit, the value of the output voltage of the first differential amplification circuit is converted into a digital value, and that by use of the average value of these digital values, the respective threshold values in the first comparison circuit and the second comparison circuit are adjusted. As a result, even when the foregoing offset occurs, the first comparison circuit can always compare the threshold value with the value, at the center of amplitude, of the output voltage of the first differential amplification circuit and can output an output signal, and the second comparison circuit can always compare the threshold value with the value, at the center of amplitude, of the output voltage of the second differential amplification circuit and can output an output signal; therefore, it is made possible that the detecting accuracy is suppressed from undergoing the effect of the variation in the respective falling times or the respective rising times of the first differential amplification circuit and the second differential amplification circuit and that the traveling direction of the magnetic moving body is accurately detected.
Moreover, with regard to such a magnetic detection apparatus as described above, there has already been disclosed a technology in which the switching device is driven based on PWM (Pulse Width Modulation) so that the first output signal and the second output signal are generated (e.g., refer to Patent Document 2).